The present invention relates to hydrogen separation by thermally cycled sorption and desorption. The invention also relates to apparatus containing hydrogen sorbents.
Purified hydrogen has long been and continues to be used in a variety of industrial processes. For example, petroleum refineries are using increasing quantities of hydrogen to meet regulatory requirements on diesel, gasoline, and other petroleum products. Hydrogen-based treating processes are expected to grow substantially because fuel regulations in North America, Europe, and other regions are becoming increasing stringent. For example, the sulfur levels in U.S. diesel fuels must decrease from the current level of 250 ppm to 15 ppm by 2007. While several options exist for lowering sulfur levels, all of commercially available processes require a hydrogen input stream.
Another major use of hydrogen is in upgrading crude oil to make gasoline. To meet the world""s increasing demand for gasoline, it has been necessary to develop poorer grades of crude oil that are denser and require hydrogenation for upgrading to gasoline.
Additionally, for more than 10 years there have been intense research and development efforts directed toward hydrogen as a clean power source for fuel cells. Compared to conventional power systems, hydrogen-powered fuel cells are more energy efficient, more robust, and less polluting. Fuel cells can totally eliminate ozone and nitrogen oxides, the most noxious precusors of smog. However, problems such as excessive cost, equipment size, and process complexity have prevented hydrogen-based fuel cell technology from replacing most conventional power sources.
The present invention provides apparatus and methods for separating hydrogen. The invention can be used, for example, to purify hydrogen formed in a steam-reforming reaction (typically a gas containing hydrogen, carbon monoxide and carbon dioxide). Compared to conventional hydrogen separation technology, many of the configurations and procedures of this invention are relatively simple, scaleable over a broad range, including small, and are amenable to cost-effective mass production.
In a first aspect, the invention provides a method of separating hydrogen gas. In this method, a hydrogen-containing gas mixture passes into a channel at a first temperature. This channel includes a sorbent within the channel that has a surface exposed to the gas. Flow through the channel is constrained such that in at least one cross-sectional area of the channel, the furthest distance to a channel wall is 0.5 cm or less. The sorption, at this first temperature, occurs at a rate of at least 0.1 mol of H2/(second)(cm3 of sorbent), where the volume of sorbent is the volume of sorbent used in the method and where the rate is averaged over the sorption phase of each cycle and the xe2x80x9cfirst temperaturexe2x80x9d is the average temperature of the sorbent (measured, for a film, at the interface of the sorbent film and the surface of the flow channel or, for a porous sorbent, within a porous sorbent) during the sorption phase. Then, energy is added to the sorbent to increase temperature of the sorbent to a second temperature that is higher than the first temperature. At the second temperature, hydrogen is desorbed and hydrogen gas is obtained. The xe2x80x9csecond temperaturexe2x80x9d is the average temperature of the sorbent (measured as above) during the desorption phase.
In a second aspect, the invention provides a method of separating hydrogen gas that includes a first step of sorbing hydrogen gas. In this first step, a hydrogen-containing gas mixture is passed into a channel at a first temperature. This channel includes a sorbent within the channel that has a surface exposed to the gas. In a second step, energy is added to the sorbent to increase temperature of the sorbent to a temperature that is higher than the first temperature. Then, in a third step hydrogen gas desorbs at a second temperature that is higher than the first temperature and hydrogen gas is obtained. In this method, the second and third steps, combined, take 10 seconds or less and at least 20% of the hydrogen sorbed in the first step is desorbed from the sorbent.
In a third aspect, the invention provides a method of separating hydrogen gas from a gas mixture, in which, in a first step, at a first temperature, a hydrogen-containing gas mixture contacts a sorbent that sorbs hydrogen. This sorbent includes a layer of Pd or Pd alloy overlying a hydrogen sorbent. Then subsequently, in a second step, energy is added to the sorbent, thus bringing the sorbent to a second temperature that is at least 5xc2x0 C. higher than the first temperature and hydrogen desorbs from the sorbent. The desorbed hydrogen obtained is in a higher purity form than the feed gas mixture.
In a fourth aspect, the invention provides a method for separating hydrogen from a gas mixture, wherein a hydrogen-containing gas mixture is passed into a first sorption region at a first temperature and first pressure, wherein the first sorption region comprises a first sorbent and wherein the sorbent temperature and pressure in the first sorption region are selected to favor sorption of hydrogen into the first sorbent in the first sorption region. Hydrogen is selectively removed from the gas mixture resulting in sorbed hydrogen in the first sorbent and a relatively hydrogen-depleted gas mixture. The relatively hydrogen-depleted gas mixture passes into a second sorption region at a second temperature and second pressure. The second sorption region comprises a second sorbent, and the temperature and pressure in the second sorption region are selected to favor sorption of hydrogen into the sorbent in the second sorption region. Hydrogen is selectively removed from the relatively hydrogen-depleted gas mixture resulting in sorbed hydrogen in the second sorbent and a relatively more hydrogen-depleted gas mixture. The second temperature is different than the first temperature. Heat is added to the first sorbent, through a distance of about 1 cm or less to substantially the entire first sorbent, to raise the first sorbent to a third temperature and hydrogen desorbs from the first sorbent. Heat is added to the second sorbent, through a distance of about 1 cm or less to substantially the entire second sorbent, to raise the second sorbent to a fourth temperature, and hydrogen desorbs from the second sorbent. Hydrogen desorbed from the first and second sorbents is obtained. The amount of hydrogen obtained from the first and second sorbents is greater than the amount that would have been obtained by operating the first and second sorbents at the same temperature, given the same total amount of added heat. Although this fourth aspect of the invention is generally applicable, it is preferred to locate the sorbent in a channel having a dimension of one cm or less that is in thermal contact with a heat exchanger to achieve rapid and efficient thermal transport.
The invention also provides an hydrogen separation apparatus in which a flow channel having an internal surface that comprises palladium (which includes a palladium alloy) on at least a portion of the internal surface. The flow channel has at least one dimension of 1 cm or less, and a heat exchanger is in thermal contact with the flow channel.
The invention further provides an hydrogen separation apparatus in which a flow channel includes an inlet, an outlet and a sorbent disposed between the inlet and the outlet. The sorbent comprises a hydrogen sorbent having a surface coating over more than 90% of the surface of the hydrogen sorbent, wherein the surface coating comprises palladium. A heat exchanger is in thermal contact with the sorbent.
In yet another aspect, the invention provides an hydrogen separation apparatus containing a flow channel with a thin film of a hydrogen sorbent. Because the film is so thin, it adheres to the apparatus even after multiple sorption/desorption cyclesxe2x80x94conditions in which conventional hydrogen sorbents (such as nickel) would crumble.
Any of the apparatus described herein can be used to separate hydrogen from a hydrogen-containing gas mixture. The invention includes this apparatus and methods using any of the apparatus described herein to separate hydrogen.
The low pressure changes involved in the temperature swing sorption (TSS) process of the invention allow very thin metal shim and foil construction, hence, allowing very low metal mass and therefore very fast cycle times. Thin-walled construction also allows high surface area per volume of TSS device, thereby producing high rates of productivity per unit volume of equipment, thereby providing a high productivity rate needed for industrial scale processing. Also, an important facet of the invention is that the high surface area per unit volume (SA/V) feature of the hardware allows the sorbent to be deposited within the device in a high surface area, thin film fashion. Hence, the hydride, once formed on the surface of the sorbent, does not need to migrate far to fully load the sorbent internal solid volume, which, in comparison, would be a slow process in conventional sorbent beds containing coarse particles that are needed to reduce pressure drop across the bed. At these conditions, Pd loads very little hydrogen. In the present invention, the thin metal film/foil/shim can be fully loaded with hydride quickly due to the short diffusion distances. A benefit to the above TSS design is that, since only very thin sorbent layers are needed, sorbent material normally considered too costly, such as Pd, can be used.
Although the ability of palladium to selectively sorb large volumes of hydrogen has been long known, we have surprisingly discovered that a fast rate of sorption and desorption occurs for hydrogen, in properly constructed apparatus or properly conducted methods, allows rapid thermal cycling to efficiently separate relatively large volumes of hydrogen gas (henceforth just hydrogen) with relatively small hardware volumes. Numerous advantages are provided by various embodiments of the present invention including: reduced cost, reduced volume of separation hardware, durability, stability, separation speed, ability to separate large volumes of hydrogen with a small volume of equipment, improved energy efficiency and reduced cost relative to packed bed or membrane technology.
The invention includes apparatus having any of the configurations indicated in the figures. However, these specific configurations are not the only means to carry out the invention and, therefore, should not be interpreted as limiting the inventive apparatus or methods. The invention also includes methods in which a hydrogen-containing gas mixture passes through any of the illustrated apparatus. For example, with reference to FIG. 5a, the invention includes a method in which a hydrogen-containing gas flows through a flow distribution sheet and the distributed flow passes into a sorbent-containing compartment.
xe2x80x9cHardware volumexe2x80x9d means the external volume of the hydrogen separator apparatus including the sum of all parts if the apparatus is not integrated in a single unit.
xe2x80x9cInternal surfacexe2x80x9d refers to the surface area in the interior of the flow channel that is exposed to flowing gas. Internal surface may be measured by appropriate techniques such as optical measurement or N2 adsorption.
xe2x80x9cSorption/desorptionxe2x80x9d refers to the total amount of gas taken in without regard to the mechanism by which the gas is taken in. In other words, xe2x80x9csorptionxe2x80x9d is the sum of adsorption and absorption.
The term xe2x80x9chydrogen-containing gas mixturexe2x80x9d means a gas mixture containing between 1 and 999,999 parts per million (ppm) hydrogen (including its isotopes) and at least one ppm of a gas other than hydrogen or its isotopes.
The term xe2x80x9cobtainingxe2x80x9d means that the hydrogen is recovered either for storage or for use in a subsequent chemical process such as combustion, fuel celloperation, chemical synthesis, etc. The term xe2x80x9cobtainingxe2x80x9d does not mean, however, that the hydrogen is used simply as a refrigerant.
The term xe2x80x9cheat exchangerxe2x80x9d means a component, or combination of components, that is capable of adding and removing heat. Preferred examples of heat exchangers include microchannels that can be switched from hot to cold fluids, electrical resistors in combination with a heat sink, and thermoelectric materials.